1. Field of the Invention
The present invention relates generally to methods and apparatus for deposition of materials on substrates, and more particularly to a method and apparatus for layer by layer deposition of thin films compatible with conventional reactor designs making ALP more amenable to a wider array of reactant sources, ALP reaction chemistries, and reactor geometries without increasing reactor and gas delivery complexity, wherein prior art pump-purge steps are avoided by providing continuously modulated gas flow and pressure, allowing all delivery valves to remain open and diverter lines closed, therefore avoiding rapid cycling of valves between on/off states.
2. Brief Description of the Prior Art
Layer by layer deposition of thin films is becoming increasingly important in semiconductor device fabrication. Layer by layer deposition offers several advantages compared to conventional chemical vapor deposition or chemical vapor epitaxy, including superior control of film thickness, improved across the wafer uniformity, ability to deposit laminated films with a small periodicity of thickness, and significantly improved film properties such as density, conformality, insulating characteristics, etc. especially as the total film thickness is scaled below 10 nm. Layer by layer deposition has been used for a variety of films including metals (Al, W, Ti, etc.), semiconductors (Si, ZnSe, III-V and II-VI compounds), oxides (SiO2, Ta2O5, Al2O3, TiO2, SrTiO3, HfO2, ZrO 2, etc.), nitrides (Si3N4, TiN, TaN, AlN), silicides (TaSiN, TiSiN) and nanolaminates of these materials. All these materials have significant industrial relevance in semiconductor devices.
In layer by layer growth, also called atomic layer processing (ALP), the film is deposited approximately a monolayer at a time; i.e. the thickness of each layer is of the order of the inter-atomic spacing and hence the term atomic layer processing. For atomic layer processing, the substrate is sequentially exposed to fluxes of reactants so that the reaction is restricted to a surface reaction between an adsorbed/chemisorbed reactant that saturates the surface, and a second gas phase reactant provided in a subsequent pulse. In this manner, the reaction is usually self-limiting once the surface absorbed reactant is consumed, resulting in a monolayer of the film. Recently it has been shown that sub-monolayers or several mono-layers can be obtained via the same technique prompting a change in nomenclature from atomic layer deposition to alternating layer deposition. The ALP technique has been used to deposit both epitaxial and nonepitaxial films. The process of depositing epitaxial films is termed atomic layer epitaxy (ALE), while the technique for depositing non-epitaxial films is traditionally termed atomic layer deposition (ALD). The vapor source reactants for ALP can be gaseous sources or generated by thermal evaporation, vaporization of liquid sources, or remote plasma dissociation.
The technique for depositing films by sequentially exposing the wafer to various reactants is well known and has been in vogue for over two decades. An important requirement for ALP is the need to isolate the reactants from each other in the gas phase. For most ALP reactions, isolating the reactants in the gas is necessary to prevent gas phase reactions between the reactant sources, and also to suppress any parasitic chemical vapor deposition that could occur if the reactants were to be simultaneously present in the gas phase. The allowable residual level of one precursor when the other is introduced is process dependent. One criterion for determining the allowable residual level of one precursor is the contribution of parasitic CVD to the overall deposition rate. For an ALP process, the contribution of the parasitic CVD to the overall deposition rate should typically be less than 10%, although higher values may be tolerable if the film properties, film uniformity and step coverage do not suffer. In fact, parasitic CVD may be used to enhance the deposition rate of the otherwise slow ALP processes. For some processes, the contribution of CVD to the overall deposition rate must be <1% to avoid particulate generation gas phase reaction. Perfect isolation of the precursors from each other during ALP is not required and in some cases may also be undesirable. Some parasitic CVD can prevent adverse reverse reactions from occurring that would otherwise etch the film being deposited. Thus a number of methods have been described in the literature for partial isolation of the precursors. One method to isolate the reactants is to confine them to different regions and move the substrates between the different regions to expose the substrates to alternating doses of the multiple reactants. This method is described in U.S. Pat. No. 4,058,430. A disadvantage of this method is that it is difficult to implement in a conventional reactor used for chemical vapor deposition, and especially for processes that require high reactant partial pressures over the substrate in order to achieve complete surface saturation. Another method to isolate the reactants is to feed the reactants in the form of pulses sequentially through separate delivery lines into a vacuum chamber containing one or more substrates. An evacuation or pumping step is performed in between consecutive pulses to evacuate the chamber of one reactant prior to introducing the next reactant. U.S. Pat. No. 4,058,430 describes this more commonly used approach of exposing a stationary substrate to alternating pulses of reactants and using an evacuation or pumping step in between the pulses to evacuate the chamber of reactants. U.S. Pat. No. 4,058,430 describes both atomic layer deposition and atomic layer epitaxy in which substrates are exposed to alternating pulses of reactants that are isolated from each other in the delivery system and in the chamber. In either of these techniques perfect isolation of the precursors is not achieved, but the residual concentration of one precursor can be decreased to an arbitrarily low level before the other precursor is introduced.
Improvements to the ALD sequence have been proposed. One improvement is to introduce a purge gas simultaneously during the evacuation step. This is generally termed a pump-purge step. The purge gas can act as a gas diffusion barrier preventing the interactions between reactants when all reactants and the purge gas follow the same flow path from the gas inlet to the chamber exhaust. This improvement is described in U.S. Pat. No. 4,389,973. This purge gas flow coupled with simultaneous evacuation of the chamber also reduces the residual concentration of the reactant to trace levels (<1%) as described in U.S. Pat. No. 6,015,590. Multiple pump-purge steps may be used to reduce reactant concentrations in the chamber even further before the next reactant pulse is introduced into the chamber. This mode of ALP is widely used because of its simplicity and ease of adaptability to a wide range of CVD reactor configurations, and is hereinafter referred to as the conventional ALP technique. For single wafer ALP reactors, this approach provides a faster way to evacuate the reactant from the chamber compared to using pumping alone. This is because simultaneous purging while pumping can reduce the concentration of the precursor to trace levels on time scales shorter than 1-2 s, while it takes considerably longer (2-5 s) if pumping alone were to be used.
A conventional ALP sequence is show in FIG. 1. Note the reactant pulsing and pump/purge steps.
Typical deposition rates for ALP are I monolayer/cycle, which translates to ˜1 Å/cycle. Each cycle according to the conventional ALP technique consists of the following steps:                Introduce a pulse of reactant 1 to form an adsorbed/chemisorbed layer on the substrate,        Simultaneously pump the reactor while purging the reactor with a pulse of a purge gas that is typically inert to reduce the residual concentration of reactant 1 in the gas phase to trace levels.        Introduce a pulse of reactant 2 into the chamber to react with the adsorbed/chemisorbed layer on the substrate resulting in the formation of the film.        Simultaneously pump the reactor while purging the reactor with a pulse of a purge gas that is typically inert to reduce the residual concentration of reactant 2 in the gas phase to trace levels.        
Thus the simplest ALP cycle consists of four distinct pulses/steps of gases. For a typical film thickness of 30-100 Å, the number of ALP cycles ranges from 30-100 cycles. Clearly, in order to achieve a high wafer throughput, a short cycle time must be achieved. Ideally a cycle time of 10 s or less allows 6 cycles/min or an equivalent deposition rate of 6 Å/min. Thus the maximum throughput for a single wafer process module that processes one wafer at a time ranges from 10-12 wph for 30 Å films and from 3-4 wph for 100 Å films. Contrast this with wafer throughputs of 20-30 wph for conventional single wafer CVD modules. For the ALP process to be cost-effective for volume production of semiconductor devices, a 5×-10× improvement in wafer throughput is necessary.
A high throughput for ALD is necessary for several reasons. Naturally, a high throughput reduces the cost of ownership for the process. More importantly, the ALD process is often used in conjunction with other high throughput processes on a vacuum integrated cluster tool. A good example is a cluster tool for a high-k gate stack that consists of modules for pre-cleaning, interfacial oxynitride growth, ALD hi-k deposition and gate electrode deposition. The module with the lowest throughput governs the throughput of such a cluster tool. Ideally the modules should be matched in throughput for maximum productivity.
Cycle times of 10s for conventional ALP also necessitate extremely short pulse and pump—purge times of the order of 2-3 s. As discussed before, cycle times longer than 10 s for single wafer ALP significantly compromise wafer throughput and thus are not production-worthy. The need to achieve short pulse and pump-purge times introduces additional hardware and process complexity including the following:                The reactant and purge gas delivery system must be designed so that reactant and purge gas transit times from the delivery system into the reactor are substantially shorter than the pulse duration. Generally this can be achieved by locating the gas delivery system close to the reactor chamber, minimizing the delivery line volume and using a carrier gas to transport the reactants to the chamber.        The reactant gas and purge gas flow components must toggle between the off state and the flow regulation state on the time scales of a fraction of second so that stable flows can be achieved during each reactant pulse. This is achievable for gaseous reactant sources but is more complicated for liquid reactant sources, remote plasma reactant sources, and other sources that respond on the time scales of several seconds. For these sources, a diverter line that delivers the flow directly into the foreline of the pump can be implemented. However, the danger of reactants mixing in the foreline of the pump leading to particulate formation must be addressed. In addition, reactants diverted into the foreline of the pump are wasted.        Repeated rapid cycling of flow components such as flow controllers and valves between the off state and the on state can result in pressure bursts in the chamber and increased particle generation/release in the flow components adversely impacting particle performance of the process. In addition, reliability of these components under constant cycling between the off state and the on state has proven to be a major concern.        For some ALP processes such as SiN using SiCl4/NH3, the reactant exposure dose for complete surface saturation exceeds 100 Torr.s; i.e., if the partial pressure of the reactant above the wafer is 1 Torr, an exposure time of 100 s is required. The alternative is to use high partial pressures of reactants, for example 10 Torr for 10 s. This is impractical in most instances involving liquid sources for the reactants since the vapor pressure of the liquid source is insufficient to achieve the desired partial pressure.        
In many ALP sequences, the reactive byproducts although present at a low concentration can drive the reaction in the reverse direction resulting in atomic layer etching. This is undesirable and must be avoided. This phenomenon typically occurs in ALP reactions that generate HF or HCl as reaction byproducts. For example, in ALP of TiN with TiCl4 and NH3, the HCl byproduct can etch the formed TiN unless a low residual background pressure of TiCl4 that suppresses the reverse reaction is present. During the deposition of compound semiconductor films such as GaAs, using sequential pulses of Ga and As, a low partial pressure of As must be present during the Ga pulse to prevent evaporation of As from the deposited film. A low partial pressure of the reactant may also be necessary to keep the reactant from desorbing during the purge step.
Deposition temperatures for ALP are typically lower than the equivalent CVD temperatures. This is because complete saturation of all surface sites is necessary which occurs at a temperature range that is slightly lower than used for the equivalent CVD process. For example, TiN can be deposited from TiCl4/NH3 using the ALP sequence at 350-450° C., while CVD process temperatures are 450-550° C. One of the drawbacks from operating at lower temperatures is that the reactions may not go to completion; which may result in non-stoichiometric films or films with higher impurity contents. For example, in films deposited with non-chlorinated precursors, residual chlorine contamination is usually an issue. Similarly for films deposited with organic precursors, carbon and hydrogen contamination are prevalent. Radical assisted ALP has been used to ameliorate these problems with reasonable success, but the exposure times are fairly long, which extends the duration of the ALP cycle degrading throughput. Using plasma annealing or plasma densification can accelerate the process, but because plasma ions have a finite penetration depth into the film, the removal of impurities is not uniform through the thickness of the film. Also, exposing the film directly to a plasma can result in plasma-induced damage to the film or underlying device structures, in addition to enhancing surface contaminants. Making a reactor chamber compatible with both ALP and plasma processing with good plasma uniformity is also a non-trivial engineering task.